Powder coating processes may generally be defined for purposes of this specification as those in which fusible coating materials are distributed over a substrate and the distributed coating materials are fused into a continuous film. Sometimes the substrates are preheated to supply the heat required to melt the coating powders, sometimes the powders are applied to cold substrates, and the powders are melted in a postheat oven, and sometimes both postheat and preheat steps are utilized.
In common practice, coating powders are based on thermoplastic or thermosetting polymeric materials but they can also include such things as ceramics and metals. In any instance, the essential requirement is that the melting point of the coating powders must be sufficiently low so that they can be melted without degrading the substrate. It also should be noted that in the case of thermosetting coating powders, a postheat is usually required to complete the cure and fully develop the properties of the coating.
Examples of commonly used powder coating processes include fluidized bed, electrostatic spraying, cloud chambers, hot flocking, fluid transport as well as various combinations of these processes.
One of the principal advantages in using powdered coatings is that no solvents or carrier liquids are required. Not only does this reduce the cost of the coating materials, but also costly volatile recovery systems needed for liquid solvent coating materials to meet the requirements of governmental environmental protection agencies are avoided.
A recurring problem, common to all powder coating systems, is that the substrate must be supported by some type of fixture when the substrate is being coated. For example, a substrate may be hung on a J-hook or, if the part is rather large, a metal wire or rod may be tackwelded to the substrate. In any instance, the point at which the substrate is supported will not be coated and the bare spot often must be touched-up with a patching compound. The importance of patching uncoated spots is more than a matter of esthetics since a bare spot presents a point where underfilm corrosion may be begin. Continous, void free coatings are also required when electrical insulation is important.
When coating substrates of moderate size and upwards, such as busbars, wall panels, or dishwasher baskets, it is usually only a matter of minor annoyance to fixture the part and patch the holding point; but when the parts are small, such as thumbtacks, bra clips, and zipper pulls, fixturing and patching make the coating process so labor intensive that it may no longer be cost competitive with solvent based systems.
A related problem, which at first may escape notice, is that powder overspray coats the fixtures and this coating must be stripped off frequently, sometimes each time the fixture is used. In extreme situations, the part and the fixture coat together and cannot be separated without cutting one piece loose from the other. Overspray coatings may be burned off the fixtures or they may be removed with solvents. In either event, an additional operation is required, and, no matter if solvents or burning is used, waste materials are generated that require collection and disposal.